Attribute selector is a selector defined by 1) the attribute set to element(s), 2) the attribute and value(s), 3) the attribute and value parts:
1a) A[title] {text-decoration: underline}
All A elements containing the TITLE attribute will be underlined
1b) A[class=name] {text-decoration: underline}
The A elements classed as 'name' will be underlined
2) A[title="attribute element"] {text-decoration: underline}
The A elements containing the TITLE attribute with a value that is an exact match of the specified value, which in this example is 'attribute element', will be underlined
3) A[title~="attribute"] {text-decoration: underline}
The A elements containing the TITLE attribute with a value containing the specified word, which in this example is 'attribute', will be underlined